Semiconductor lasers used in conventional laser transmitters have an active region which is of the order of two micrometers in cross-section. In an optical transmission system using optical fibers as the transmission medium, the light emerging from the active region at one facet of the laser, the "front" facet, needs to be coupled into the core of an optical fiber. If the fiber is a single mode fiber, the core is only about five micrometers to ten micrometers in diameter.
For many present applications stabilizing the output power of a light source is sufficient even though the level at which the power output is stabilized differs arbitrarily from device to device. Simple bi-directional optical links, and optical disk read heads are examples of such applications. Alignment between the laser and a photo-detector monitoring the output of the light source is much less critical for these purposes, as a slow speed photo-detector has a typical diameter of two hundred fifty (250) micrometers.
An optical read/write head for optical recording and reproduction is described in published European Patent Application EP-A- No.0199565 (SONY Corp.). The optical read/write head comprises a laser diode transmitter, a photo-diode receiver, and a semi-silvered prism providing a semi-reflecting mirror. Light from the laser transmitter is partially reflected by the semi-reflecting mirror onto the optical recording medium. The modulated beam reflected by the medium is partially transmitted by the same semi-reflecting mirror to the photo-diode of the receiver. The light output of the laser is monitored either by sensing directly the light emerging from the back facet of the laser, or by monitoring a portion of the light output of the front facet of the laser, which is transmitted through the semi-reflecting mirror formed on the prism, and reflected again by a fully reflecting portion of that prism onto a monitoring photo-diode.
A similar arrangement using semi-reflecting mirrors is described in published French patent application FR-A No. 2581768 (Thomson-CSF), in connection with a bi-directional optical communications link. Japanese Appln. No. 57-96582 discloses the use of light reflector in connection with an inspection device for a semiconductor laser.
In each case the absolute output power of the laser is, within fairly broad limits of specified laser performance, of secondary importance, and may vary from device to device over a considerable range without adverse effects.
However, mainly in newer applications such as optical networks using a plurality of transmitters and/or receivers, for example, the power output of individual transmitters needs to fall within much tighter tolerance limits. Some of the problems encountered in trying to achieve better control over the absolute level of output power of an optical transmitter arise from tolerances in semiconductor device manufacture, of both the laser and the monitoring detector.
Various other difficulties related to the manufacture and assembly of laser transmitters, such as ensuring adequate heat dissipation from the active area of the laser and providing convenient bonding areas for electrical connections to the laser and to the photo-detector, for example, will be well known to the person skilled in the art.
Where only small numbers of transmitters are required, it is obviously possible to select a few suitably matched transmitters from among the total manufacturing output in order to fulfill special performance requirements. For large scale supply of such transmitters, this is clearly not a practical approach. The present invention aims to provide an optical transmitter assembly which enables the photo-detector electrical output to be more reliably precalibrated with respect to the optical output of the laser, and which thereby avoids or at least mitigates some of the aforementioned problems.
The optical power coupled into the fiber depends on the alignment and distance between the light source and the fiber. On occasion, the coupling between source and fiber may be deliberately kept below optimum in order to limit the maximum optical power transmitted through the fiber. The present invention further aims to provide a transmitter assembly in which the electrical output of the photodetector can be calibrated to the optical power coupled into the fiber rather than the optical output power of the light source.